Rapid advances are occurring for making and using various electronic devices especially display devices that are used for various communication, financial, and archival purposes. For such uses as touch screen panels, electrochromic devices, light emitting diodes, field effect transistors, and liquid crystal displays, electrically-conductive films are essential and considerable efforts are being made in the industry to improve the properties of those electrically-conductive films.
There is a particular need to provide touch screen displays and devices that contain improved electrically-conductive film elements. Currently, touch screen displays use Indium Tin Oxide (ITO) coatings to create arrays of capacitive areas used to distinguish multiple point contacts. ITO coatings have significant short comings. Indium is an expensive rare earth metal and is available in limited supply from very few sources in the world. ITO conductivity is relatively low and requires short line lengths to achieve adequate response rates. Touch screens for large displays are broken up into smaller segments to reduce the electrically-conductive line length to an acceptable resistance. These smaller segments require additional driving and sensing electronics. In addition ITO is a ceramic material, is not readily bent or flexed, and requires vacuum deposition with high processing temperatures to prepare the electrically-conductive layers.
Silver is an ideal electrical conductor having electrical conductivity of 50 to 100 times greater than ITO. Silver is thus used in many commercial applications and is available from a number of sources. It is highly desirable to make electrically-conductive film elements using silver as the source of electrical conductivity.
Numerous publications describe the preparation of electrically-conductive films formed by reducing a silver halide image in silver halide emulsions in the form of electrically-conductive grid networks having silver wires having sizes of less than 10 μm. Various efforts have been made to design the silver halide emulsions and processing conditions to optimize such electrically-conductive grid networks and the methods for making them.
For example, improvements have been proposed for providing electrically-conductive grid patterns from silver halides by optimizing the silver halide emulsions as well as finding optimized processing solutions and conditions to convert latent silver images into silver metal grid patterns. The precursors used to provide the electrically-conductive grid patterns can comprise one or more silver halide emulsion layers on opposing sides of a transparent substrate.
While these processes and articles can provide desired electrically-conductive films, optimizing the design of both the precursors and processing procedures requires considerable effort in order to achieve the exacting features required in electrically-conductive films to be incorporated into touch screen displays.
Other industrial approaches to preparing electrically-conductive films or elements have been directed to formulating and applying photocurable compositions containing dispersions of metal particles such as silver metal particles to substrates, followed by curing the photocurable components in the photocurable compositions. The applied silver particles thus act as catalytic (seed) particles for electrolessly plated electrically-conductive metals. Useful electrically-conductive grids prepared in this manner are described for example in U.S. Patent Application Publications 2014/0071356 (Petcavich) and 2015/0125596 (Ramakrishnan et al.). Other details of a useful manufacturing system for preparing conductive articles especially in a roll-to-roll manner are provided in WO 2014/070131 (Petcavich et al.).
Using these known methods and photocurable compositions containing silver particles can be printed and cured on suitable transparent substrates for example, a continuous roll of transparent and flexible polyester film and then carrying out electroless plating. However, these methods require high quantities of silver particles dispersed within the photocurable compositions in a uniform manner so that coatings or printed patterns have sufficiently high concentration of seed catalytic sites. Generally, this cannot be achieved without carefully designed dispersants that can be expensive to make or purchase and tedious or difficult to use to provide sufficient reproducibility in a high speed manufacturing operation. Without effective dispersing, silver particles can readily agglomerate in the curable compositions, leading to less effective and uniform catalytic metal patterns and electroless plating. The resulting electrically-conductive films or elements would lack the desired electrical conductivity or durability, or both.
U.S. Patent Application Publication 2007/0261595 (Johnson et al.) describes a method for electroless deposition on a substrate that uses an ink composition containing silver as a reducible silver salt and filler particles.
U.S. Pat. No. 7,682,774 (Kim et al.) describes photosensitive compositions comprising silver fluoride organic complex precursors as catalyst precursors as well as a polymer derived from a monomer having a carboxyl group and a co-polymerizable monomer.
Stannous (II) salts are effective reducing agents for silver and silver mirrors are routinely produced on glass surfaces using stannous chloride as sensitizer (see Chem. Educ. 2011, 88, 274-275 and J. Aus. Cer. Soc. 2013, 49, 62-69).
U.S. Pat. No. 3,202,513 (Thommes) describes the use of water-soluble stannous salts such as stannous chloride, stannous palmitate, and stannous chloride in photopolymerizable compositions that are stated to be useful in the preparation of printing relief elements and printed circuits.
U.S. Pat. No. 6,390,636 (Takahagi et al.) describes the use of aqueous hydrochloric acid solution of stannous chloride and palladium chloride as activating treatment agent to form silver mirrors.
Electroless plating had been used for a long time as an industrial process to provide conductive or decorative coatings on surfaces. Before electroless plating, a palladium (Pd) activation step that introduces the catalytic sites onto the surface had to be carried out. The conventional method of surface catalysis involved immersion of the surface in a sensitizing solution containing SnCl2/HCl solution followed by immersion in activating solution containing PdCl2/HCl solution. This induces a redox reaction whereby Sn2+ is oxidized to Sn4+ and Pd2+ is reduced to Pd(0). This Pd(0) is an active catalyst for electroless deposition of metals. This conventional “two-step” activation process in some cases has been replaced by “one-step” process that uses a colloidal mixture of SnCl2 and PdCl2 (see Lee et al. Latest Trends in Environmental and Manufacturing Engineering). The particle size of the palladium colloid thus formed is around 1-2 nm. Growth beyond that point is limited by the spontaneous formation of a tin chloride shell around each palladium particle, which must be removed as this shell inhibits growth of metal deposit.
Stannous chloride activation is also used in the manufacture of printed circuits. This manufacturing process utilizes an electroless metal deposit on a dielectric substrate either as a uniform surface coating or in a predetermined pattern. This initial electroless deposit is usually thin and is further built up by electroplating. The usual process for achieving the electroless metal coating on non-conductive or semi-conductive substrates such as polymeric films comprises treating the surface by immersion in a bath containing stannous chloride or another stannous salt; seeding or catalyzing to provide catalytic nucleating centers by immersion in a salt of a metal catalytic to the deposition of the desired metal coating such as silver nitrate or the chlorides of gold, palladium, or platinum, these metal ions being reduced to catalytic metal nucleating centers by the stannous ions adsorbed on the substrate or by reducing agents contained in the electroless metal deposition bath; and thereafter depositing the desired metal, such as copper, nickel, or cobalt by treating the catalyzed surface with a salt of the desired metal plus a reducing agent therefor.
As described above, the effective solutions for catalytic sensitization of the surface requires mixing a palladium salt, such as palladium chloride, and a tin salt such as stannous chloride in aqueous hydrochloric acid. These methods or materials are not suitable for coating or flexographic printing processes especially in high speed roll-to-roll manufacturing operations.
Advances in the art are provided using various reducible silver complexes have been designed so that upon reduction, the resulting silver metal provides catalytic sites for electroless plating. For example, non-aqueous metal catalytic compositions have been designed to include suitable silver complexes containing reducible silver ions and silver ion reducing components.
There is a further need to provide additional improvements by improving the generation of catalytic silver particles.